The field that this invention relates to control systems for downhole valves and more particularly subsurface safety valves.
Subsurface safety valves principally are designed around the concept of a spring actuated flow tube which is hydraulically operated so that when the flow tube is shifted downwardly it displaces a flapper off of a seat by rotating it ninety degrees leaving the central passage in the flow tube open. Reversal of these movements allows the spring loaded flapper to rotate ninety degrees against the seat and seal off the flow path. Control systems to actuate the flow tube into a downward motion to open the subsurface safety valve have come in a variety of configurations in the past. One of the design parameters is obviously the ability to shift the flow tube to open the subsurface safety valve. Another design parameter is to allow the hydraulic control system to have a fail safe operation in the event there are malfunctions in the system. Yet another criteria is to make such a system small and uncomplicated to ensure its reliability over an extended period of time in which the subsurface safety valve may be in operation in a well.
One of the problems of control system designs particularly in applications where the subsurface safety valve is set deeply such as depths below ten thousand feet from the surface is that the power spring on the flow tube may be required to support the hydrostatic pressure in the control lines to the dynamic piston which moves the flow tube. Since the required stroke of the flow tube is quite long, springs that can resist hydrostatic at such depths become very cumbersome. Accordingly one of the objects of the present invention is to provide a system for hydraulic flow tube control where the power spring requirements are such that it is not mandatory to be able to support the control line hydrostatic pressure in the control system. Another objective of the present invention is to eliminate charged chambers usually filled with nitrogen that have been employed in some of the designs used in the past. Another objective of the present invention is to offer a simplified system which can be easily modified for a variety of depths and can provide reliable service over a long period of time while at the same time being simple to construct and simple in its operation.
Control systems typical of those previously used can be readily understood from a review of U.S. Pat. Nos. 5310004, 5906220, 5415237, 4341266, 4361188, 5127477, 4676307, 466646, 4161219, 4252197, 4373587, 4448254, 5564501 as well as U.K. Applications 2159193, 2183695, 2047304.
The hydraulic control system for operating a flow tube in a subsurface safety valve is disclosed. An isolation piston is used in conjunction with an operating control line and an engagement control line. Both control lines run from the surface. The isolation piston is spring loaded to equalize pressure across a dynamic piston to allow the flow tube to be shifted by a power spring to allow in turn the subsurface safety valve to close. Application of pressure on the engagement control line directs pressure applied through the operating control line to the top of the dynamic piston thus shifting the flow tube downwardly to open the subsurface safety valve. In an alternative embodiment, a coaxial control line directs fluid to the top of the dynamic piston and additionally to a parallel path leading to the bottom of the dynamic piston where a control valve is mounted. The control valve can be actuated hydraulically, electronically or other ways such that when it is closed the pressure applied to the dynamic piston shifts the flow to open the subsurface safety valve. A loss of signal to the control valve equalizes the dynamic piston allowing the flow tube to shift.